This application claims the priority benefit of Taiwan application serial no. 91119480, filed Aug. 28, 2002.
1. Field of the Invention
The invention relates in general to a light-emitting device display technique, and more particular, to a driving technique of an active matrix organic light-emitting diode (AMOLED) to increase the stability of threshold voltage as a function of time.
2. Related Art of the Invention
Following technical advancement, video products, and particularly digital video or image apparatus, have become products commonly seen in our daily lives. Among digital video or image apparatus, the display is a very important device for displaying relative information. The user can read information from the display, or further control operation of the apparatus.
To comply with modern life style, the video or image apparatus has been integrated with lighter weight and smaller volume. The conventional cathode ray tube, though good from a certain aspect, has been replaced by the flat panel display due to the concerns of volume occupancy and power consumption. The currently available flat panel display in the market includes the liquid crystal display and active matrix organic light emitting diode, for example.
The technique of liquid crystal display has been developed for years without a significant breakthrough. The active matrix organic light-emitting technique is a new technique and likely to become the main stream of display. The characteristics of the active matrix organic light emitting diode include using thin-film transistor (TFT) technique to drive the light-emitting diode, and forming the driving integrated circuit (IC) on the panel directly. Therefore, the requirements of being thin, light, short and small, and low cost are met. The active matrix organic light-emitting diode is suitable for use in cellular phones, personal data assistants, digital cameras, palm pilots, portable DVD players, and the vehicle navigation system. The future application of the active matrix organic light-emitting diode includes large-scale flat panels such as for computers and flat panel televisions.
For digital displays, the display screen is formed of a plurality of pixels arranged as an array. To control individual pixels, a scan line and a data line are used to apply operation voltage to the selected pixels, such that the data of the selected pixels are displayed. FIG. 1 shows a schematic circuit diagram of a conventional driving circuit of an active matrix organic light-emitting diode. The driving circuit includes a transistor 100 and a transistor 102. The transistors 101 and 102 are thin-film. transistors (TFT). The gate of the transistor 100 is coupled to a scan line to receive a scan voltage Vscan at an appropriate pulse, and the source region thereof is to receive a data voltage Vdata from a data line at this pulse. The drain region of the transistor 100 is coupled to the gate of the transistor 102. Normally, the source region and drain region of the transistors 100 and 102 are interchangeable. A storage capacitor 106 is connected between the gate of the transistor 102 and a ground voltage. The drain region of the transistor 102 is coupled to a source voltage VDD. The source region of the transistor 102 is coupled to an anode of an active matrix organic light-emitting diode 104 in series. The active matrix organic light-emitting diode 104 further has a cathode coupled to a relative low voltage VSS.
The operation of the driving circuit as shown in FIG. 1 is described as follows. When the gate of the transistor 100 is conducted by receiving the scan voltage Vscan from the scan line, the data voltage Vdata is input to the gate of the transistor 102 to conduct via the transistor 100. Thereby, the transistor 102 is also conducted. The source voltage VDD is then applied to the organic light-emitting device 104 to emit a light. The transistor 102 is typically referred as a driving device. During circuit operation, the voltage Vscan will be input to the transistor 100 with a predetermined frequency via the scan line. The time period between two consecutive pulses is referred as a frame, and a predetermined image data block will be input to the corresponding pixel within a frame. When the transistor 100 is activated by a clock pulse of the scan voltage Vscan, the transistor 102 is activated by the data line Vdata. The data voltage Vdata is stored in the storage capacitor 106 to maintain the activation of the transistor 102.
Therefore, the organic light-emitting device 104 is switched on in any frame. It is only that different gray scale display results according to the data voltage in different frames. In other words, in the traditional design, the light-emitting device of TFT active matrix organic light-emitting diode is continuously illuminating. This luminescent method meets the image display effect and prevents the screen from flashing. However, as the light-emitting device is continuously driven, the transistor 102 is maintained at an on state all the time. For a normal transistor 102, particular the thin-film transistor 102, the long operation time will cause change of the characteristics. For example, the threshold voltage is increased with operation time. As shown in FIG. 2, this affects the luminescent status of the light-emitting device. For example, the luminance or chroma is changed. The effect caused by the shift of the threshold voltage can be expressed by the following relationship.
When the light-emitting device 104 is activated, the driving currentD can be presented by equations (1) and (2) as:                               I          D                =                              1            2                    ⁢                                    k              ⁡                              (                                                      V                    gs                                    -                                      V                    th                                                  )                                      2                                              (        1        )                                          I          D                =                              1            2                    ⁢                                    k              ⁡                              (                                                      V                    G                                    -                                      V                    S                                    -                                      V                    th                                                  )                                      2                                              (        2        )            
In the above equations, k is a characteristic constant of thin-film transistor. From equations (1) and (2), if the threshold voltage is increased with time, the driving current/D flowing through the organic light-emitting device 104 is decreased, and consequently, the luminance is decreased. The lifetime is also determined by the luminance. Therefore, the variation of threshold voltage Vth is crucial to the organic light-emitting device 104.
The present invention provides a driving circuit for a light-emitting device. The driving circuit can maintain the threshold voltage of a driving transistor at a stable value after long operation time of image display. Therefore, the product display quality is enhanced.
The present invention further provides a driving circuit for a light-emitting device that receives a normal scan line signal and an additional scan line signal with a delay to the normal scan line signal. When the driving circuit of the light-emitting device is activated by the additional scan line signal, the normal image data voltage is replaced by a discharge low voltage. Thereby, the driving transistor is switched off, and the threshold voltage Vth is reset to the initial value.
The driving circuit of the light-emitting device provided by the present invention is suitable for using in an active matrix organic light-emitting diode. The driving circuit comprises a driving transistor, of which a gate is coupled to a node. The light-emitting device is coupled to the driving transistor in series to form a light-emitting path. The light-emitting path is connected between a system high voltage and a system low voltage. When the driving transistor is switched on, the light-emitting device is driven by the system high voltage to be illuminated. The driving circuit also comprises a maintaining capacitor coupled to the node to maintain the on state of the driving transistor. The driving circuit further comprises a system driving path which includes a first transistor and a second transistor connected to the node in series. The first transistor has a gate to receive a first scan clock pulse and the second transistor has a gate to receive a second scan clock pulse. The first clock pulse and the second clock pulse have the same frequency. The second scan clock pulse is delayed from the first scan clock pulse by a delay time.
When the first transistor is activated by a plurality of continuous pulses of the first scan clock pulse, a data voltage corresponding to a frame is input to the node to control the activation of the driving transistor, so as to perform an image display. When the second transistor is activated by a plurality of continuous pulses of the second scan clock pulse, a switch-off voltage is input to the node to switch off the driving transistor.
The switch-off voltage as mentioned above is a negative voltage allowing the driving transistor to be switched off, and the capacitor to discharge to a lower voltage level.
The present invention further provides a driving circuit of a light-emitting device. The driving circuit includes a driving transistor. The light-emitting device is coupled to the driving transistor to form a light-emitting path. The on/off state of the driving transistor determines the on/off state of the light-emitting path. The driving circuit further includes a first transistor, a second transistor and a maintaining capacitor. The first transistor has a source region coupled to a data line, a drain region coupled to the gate of the driving transistor, and a gate coupled to a first scan line. The second transistor has a source region coupled to a reference low voltage, a drain region coupled to the gate of the driving transistor, and a gate coupled to a second scan line. The clock pulses of the first and second scan lines have the same frequency. The clock pulse of the second scan line is delayed from that of the first scan line by a delay time. The maintaining capacitor is coupled to the gate of the driving transistor to maintain a voltage state.
The reference low voltage is a negative voltage switching off the driving transistor and discharging the maintaining capacitor to a low voltage level.
The present invention further provides a driving method for a light-emitting device using a driving circuit that comprises a light-emitting unit and a control transistor. The control transistor is controlled by a scan line and a data line to output a control signal to an input terminal of the light-emitting unit. The method includes providing a reset device to output a voltage signal via a clock pulse. The clock pulse of the reset device has the same frequency of a clock pulse of the scan line, however, is delayed therefrom by a delay time. According to the clock pulse of the reset device, the voltage signal is output to the input terminal of the light-emitting device to temporarily stop the light-emitting device from illuminating.